Support jobs parameter in train command

[osma]

This PR adds a new jobs parameter for the annif train command, similar to the one already implemented in eval and hyperopt commands. The intent is to make it easier to control the amount of threads/CPUs used for training, for those backends that can make use of parallel processing during training.

The PR adds this support to the fasttext and Omikuji backends, for which this was simple to implement.

Adding as draft PR for now, since this needs more testing and QA tool checks.

This is a sideshoot of PR #511 which will benefit from this, but I thought it was cleaner to implement it as a separate PR.

[codecov]

Codecov Report

Merging #512 (884f01f) into master (483f047) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #512   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          82       82           
  Lines        5774     5786   +12     
=======================================
+ Hits         5746     5758   +12     
  Misses         28       28           

Impacted Files	Coverage Δ
annif/backend/backend.py	100.00% <100.00%> (ø)
annif/backend/ensemble.py	100.00% <100.00%> (ø)
annif/backend/fasttext.py	97.82% <100.00%> (+0.04%)	⬆️
annif/backend/maui.py	100.00% <100.00%> (ø)
annif/backend/mllm.py	100.00% <100.00%> (ø)
annif/backend/nn_ensemble.py	99.24% <100.00%> (ø)
annif/backend/omikuji.py	98.76% <100.00%> (ø)
annif/backend/pav.py	98.90% <100.00%> (ø)
annif/backend/stwfsa.py	100.00% <100.00%> (ø)
annif/backend/svc.py	100.00% <100.00%> (ø)
... and 7 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 483f047...884f01f. Read the comment docs.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[osma]

Fixed test coverage. Tested this with fasttext and omikuji backends; by default both will use all CPUs, but setting the --jobs parameter controls the number of CPUs as it should.

Ready for review.